1. Field of the Invention
This invention relates to an ArF excimer laser device for exposure, and more particularly, an ArF excimer laser device for exposure in which a laser operation having a long laser pulse width is carried out.
2. Description of the Related Art
In order to obtain a fine formation and a high degree of integration of a semiconductor integrated circuit, it is necessary to improve the high-resolution capacity in a projection-type exposure device. Accordingly, a short wavelength of exposure light radiated from an exposure light source was promoted, and an ArF excimer laser device has effectively been applied as a next-generation semiconductor exposure light source.
In the ArF excimer laser device, an electrical discharge is generated within the laser chamber in which laser gas, a gas mixture comprised of rare gases such as argon (Ar) gas and neon (Ne) acting as a buffer gas and fluorine (F2) gas or the like, is hermetically enclosed at several 100 kPa, and the laser gas acting as laser medium is excited.
Since the spectral width of the laser beam in the ArF excimer laser device is wide enough to have a value of about 400 pm, it becomes necessary to obtain a narrow band formation of the spectral width of less than 1 pm in order to avoid the problem of a chromatic aberration in the projection optical system of the exposure device. The narrow band formation of the spectral line width is realized, for example, by arranging a narrow band formation optical system comprised of a beam enlargement prism and a refraction grating within the laser resonator.
However, the ArF excimer laser device is made such that the main oscillating wavelength is 193.3 nm which is shorter than the main oscillating wavelength of 248 nm of a KrF excimer laser device which is presently commonly used as a light source for exposure. Due to this fact, damage to the crystal acting as the glass material used in a projection lens system of the exposure device, such as a stepper and the like, is high as compared with when a KrF excimer laser device is used, and so the problem of a short lifetime of the lens system arises.
There are generally two types of crystal damage, namely, the formation of a colored spot caused by absorption of two photons and a compaction (an increased refractive index). The former may become apparent in the reduction of the transmission factor, the latter in a reduction of the resolution of the lens system. This influence is inverse proportional to the laser pulse width Tis defined by the following equation, provided that the energy of the laser pulse is kept constant:
Tis=(∫T(t)dt)2/∫(T(t))2dtxe2x80x83xe2x80x83(1)
where T(t) is the time-dependent laser shape.
In the following, the definition of this laser pulse width Tis will be described. If it is assumed that the damage of the optical element is generated by absorption of two photons, the damage is proportional to the square of the intensity, so that a damage D accumulated per one pulse is given by the following equation:
D=k∫(P(t))2dtxe2x80x83xe2x80x83(2)
where k is a material constant and P(t) is the time-dependent laser intensity (MW).
The laser intensity P(t) can be divided into time and energy according to the following equation:
P(t)=Ixc2x7T(t)/∫T(txe2x80x2)dtxe2x80x2xe2x80x83xe2x80x83(3)
where I is the energy (mJ) and T(t) is the time-dependent laser shape.
When P(t) is integrated on a time-basis to obtain I, and in case where an ArF excimer laser (to be described later) is used for exposure, the value of I is 5 mJ.
In this case, when the equation (3) is combined with the equation (2), the damage D is expressed by the following equation:
D=kxc2x7I2∫(T(t)/∫T(txe2x80x2)dtxe2x80x2)2dt=kxc2x7I2∫(T(t))2dt/(∫T(t)dt)2xe2x80x83xe2x80x83(4)
And combination with equation (1) leads to
D=kxc2x7I2/Tisxe2x80x83xe2x80x83(5)
From this equation (5), since kxc2x7I2 is kept constant (I is kept constant), the pulse width Tis, in inverse proportion to the damage D, is defined by the equation (1).
This laser pulse width may reflect an actual pulse width, and when the pulse width is kept the same, the value of Tis is extended until it approximately has a rectangular shape.
The narrow band ArF excimer laser device for exposure that is now available on the market is generally applied such that the repetition rate of the oscillating operation (hereinafter called a repetition rate) is 1 kHz and the laser beam output is 5 W. It is necessary to obtain a laser pulse width Tis of 30 ns or more in order to avoid damage to the optical system installed in the exposure device.
As described above, in order to reduce the damage to the optical system, it is required to extend the laser pulse width Tis (long pulse generation), although this long pulse generation is also required in view of the following points.
In the projection exposure device, the resolution R of an image projected onto a workpiece, such as a wafer coated with a photoresist, through a projection lens and the focusing depth DOF are expressed by the following equations:
R=k1xc2x7xcex/NAxe2x80x83xe2x80x83(6)
DOF=k2xc2x7xcex(NA)2xe2x80x83xe2x80x83(7)
where k1 and k2 are coefficients reflecting characteristics of the resist and the like, xcex is the wavelength of the exposure light radiated from the exposure light source and NA is the number of apertures.
In order to improve the resolution R, as apparent from the equation (6), the wavelength of the exposure light has to be short and a high NA value is desirable; although, correspondingly, as indicated in the equation (7), the focusing depth DOF is reduced. Due to this fact, since the chromatic aberration is strongly influenced, it is necessary to narrow the spectral line width of the exposure light. That is, it is further required to achieve a narrower band formation of the spectral line width of the laser light radiated from the ArF excimer laser device.
Proc.SPIE Vol.3679 (1999) pp.1030 to 1037 describes that, when the laser pulse width is extended, the spectral line width of the laser beam is narrowed, and actually, the experiments performed by the present inventor proved this fact. That is, in order to improve the resolution R, it is further required to narrow the band formation of the spectral line width of the laser beam, and so it is required to have a long pulse generation of the laser pulse width.
As described above, in order to avoid damage to the optical system of the exposure device and improve the resolution, it has been required to ensure a long pulse generation of the laser pulse width Tis. It is well known in the art that the laser pulse width Tis is dependent on the fluorine gas concentration in the laser gas enclosed in the laser chamber (see Proc. SPIE Vol.3679 (1999) pp. 1030 to 1037 mentioned above) and so the concentration of fluorine gas is adjusted to enable the laser pulse width Tis to attain a long pulse generation with Tisxe2x89xa730 ns.
In recent years, there has been a demand for a high repetition rate with regard to the ArF excimer laser device which is strongly demanded as a light source for next-generation semiconductor exposure applications in order to accomplish a high through-put during exposure processing. The present inventor has developed an ArF excimer laser device for exposure which can be operated with a repetition rate of more than 3 kHz in order to serve the mentioned demand.
The concentration of fluorine gas in the laser gas enclosed in the laser chamber was changed in order to attain a laser pulse width Tis satisfying the relation of Tisxe2x89xa730 ns. As long as the repetition rate did not exceed 2 kHz, the long pulse generation could be attained by adjusting the concentration of fluorine gas in the laser gas to attain a relation of Tisxe2x89xa730 ns. However, in the case that the repetition rate exceeded 2 kHz (for example, 3 kHz), whatever concentration of fluorine gas in the laser gas was used, it was not possible to attain a long pulse generation fulfilling the relation of Tisxe2x89xa730 ns.
The present invention has been invented in view of the problems of the prior art, and it is an object of the invention to provide an ArF excimer laser device for exposure in which the laser pulse width is 30 ns or more even in case the repetition rate exceeds 3 kHz.
The present inventor particularly investigated the argon gas in gases constituting the laser gas enclosed in the laser chamber, and, as a result of continuous investigations, discovered that when the ArF excimer laser device is operated with a repetition rate of 3 kHz or more, the laser pulse width Tis is dependent on the concentration of the argon gas.
In addition, it has been found that in the case of repetition rates of 2 kHz or less, the laser pulse width Tis is not dependent on the concentration of argon gas.
The repetition rate in the prior art ArF excimer laser devices usually available on the market was mainly 1 kHz and the repetition rate realized by the ArF excimer laser devices available in research institutes was about 2 kHz. Due to this fact, as described above, the dependency of the argon gas concentration on the laser pulse width Tis was not discovered resulting in that only the concentration of fluorine gas was adjusted.
The present inventor discovered that, contrary to the prior art approach, a laser pulse width Tis of at least 30 ns could be realized with a repetition rate exceeding 3 kHz by employing a new approach of adjusting the concentration of argon gas.
That is, the ArF excimer laser device for exposure of the present invention which achieves the aforesaid object is comprised of a laser chamber filled with a laser gas composed of fluorine gas, argon gas and at least one rare gas other than argon gas and a high voltage pulse generating device for generating a high voltage pulse electrical discharge within the laser chamber, exciting said laser gas and discharging laser beams so as to perform a quickly repeated oscillating operation of more than 3 kHz. The laser operation is carried out in a half-period for starting an electrical discharge oscillating current waveform of a pulse of reversed polarity generated by said high voltage pulse generating device and at least two subsequent half-periods. The pressure of the laser gas in the laser chamber is 2.5 to 3.5 atm; the fluorine concentration of the laser gas is at most 0.12%; and the argon gas concentration of said laser gas is from 2% to 3%.
As regards the percentages given in connection with the gas concentrations, if not stated otherwise, these are generally in vol.-%.
In addition, it is preferred that the rare gas other than argon gas is a gas mixture containing neon gas and xenon gas, the concentration of xenon gas being 5 to 15 ppm.
In the present invention, a laser operation is carried out in an initial half-period of an electrical discharge oscillating current waveform of a pulse of reversed polarity generated by the high voltage pulse generating device and in at least two subsequent half-periods. The pressure of the laser gas in the laser chamber is 2.5 to 3.5 atm, the fluorine concentration of the laser gas is 0.12% or less, and the argon gas concentration in the laser gas is between 2% and 3%. As a result, it is possible to realize an ArF excimer laser device for exposure with a laser pulse width Tis of 30 ns or more, even if the repetition rate is 3 kHz or more. Here, the concentration of argon gas is the partial pressure of the argon gas relative to the pressure within the laser chamber.
As apparent from the foregoing description, the ArF excimer laser device of the present invention is constituted such that the laser operation is carried out in the first half period of the electrical discharge oscillating current waveform of a pulse having a reversed polarity generated by the high voltage pulse generating device and in two subsequent half-periods, wherein the pressure of the laser gas in the laser chamber is set to 2.5 to 3.5 atm, the fluorine concentration of the laser gas is 0.12% or less and the argon gas concentration of the laser gas is 3% or less, whereby it is possible to realize an ArF excimer laser device for exposure with a laser pulse width Tis of 30 ns or more, even if the repetition rate is 3 kHz or more.
In addition, if the argon gas concentration is 2% or more, it is possible to set the energy per pulse to a value higher than 5 mJ, which is desirable in view of the performance of the exposure device and the performance of the resist coated onto a wafer.
Further, if the rare gas other than argon gas is a gas mixture containing neon gas and xenon gas, and the xenon gas concentration is 5 to 15 ppm, it is possible to expand the range of argon gas concentration to where the laser output energy per pulse is at least 5 mJ.